Disc brakes have been utilized in automotive service braking applications for automobiles and light trucks for many years. Such brakes have gained such wide acceptance in these applications because of superior performance to conventional drum brakes that almost all such vehicles utilize disc brakes for some or all wheels of the vehicle. Disc brakes have gained less acceptance for service braking applications in connection with air actuated brake systems used on heavy trucks, buses and other heavy wheeled vehicles.
That disc brakes have not gained greater acceptance in connection with heavy vehicles resides partially in the fact that during braking applications when the vehicles are heavily loaded large forces must be applied to the brake shoes in order to obtain satisfactory braking service. Because of the large forces involved in braking heavy vehicles, the temperature at the interface of the friction material and the wheel drum or rotor may exceed 1800.degree. F. By design, the friction braking material has a thermal conductivity which allows much of the heat generated at the braking interface to be absorbed into the friction material. This design reduces the peak and operating temperatures at the braking interface and thereby reduces brake fade during heavy, frequent and prolonged braking applications.
Because of the relatively high temperatures developed at the friction material/brake drum interface of heavy vehicle braking systems, the brake pads utilized in disc brake systems under some conditions have been found to separate from a backing plate which conventionally attaches the friction material to a disc brake caliper.
In disc brake pads utilized in early disc brake systems, the friction material was riveted to a backing plate which preferably was steel and the assembly was mounted within a disc brake caliper. One problem associated with employing rivets to fasten the friction material to the backing plate was that by necessity a portion of the rivet had to reside within the friction material, usually a distance of between 0.125 and 0.500 inches, which rendered the portion of the friction material between the outer end of the rivet and the backing plate unusable. Accordingly, later developments led to the use of structural adhesives to bond friction material to backing plates for disc brake applications. The bonded friction material brake pads and shoes had the advantage that none of the friction material above the surface of the backing plate was sacrificed and the structural adhesives which formed the bond, in some applications, had a greater resistance to shear failure of the friction material with respect to the backing plate than rivets.
However, despite the advantages inherent in bonded brake shoes or pads over riveted shoes or pads, the use of bonded disc brake pads in disc brake systems for heavy trucks and other heavy wheeled vehicles has seen the debonding of friction material elements from backing plates under some applications of extreme thermal and mechanical loading. In an effort to prevent detachment of the friction material from the backing plate, various mechanical attachments have been employed including extruding the friction material into holes formed in the backing plate when the friction material preform is being subjected to heat and pressure during the curing operation. Although disc brake pads for use in disc brake systems on heavy vehicles have improved, detachment of the friction material from the backing plate continues to remain a problem in these vehicles where extreme thermal loads and mechanical loads regularly are encountered.
Accordingly, it is desirable to provide a disc brake pad for use in a disc brake system on heavy wheeled vehicles in which the friction material will remain attached securely to the backing plate of a disc brake pad despite the extremely high thermal loads and mechanical loads these systems encounter during severe braking applications.